Summer auctions: Old Master Week in London

Auction viewings are under-rated. Collectors and dealers go, but the interested public neglects these rare chances to see things that might not be on public display again for a generation. It's not just the museum-quality masterpieces that are worth seeing. Seeing lots of mundane pictures helps develop a feel for relative quality and gives a sense of art history's mountains, as well as the peaks that museums select for us. I started going to viewings as a teenager and I still love them.

Old masters are even more under-rated than auction viewings. It's just inexplicable to me that they are so cheap in a world that's so rich. Whenever a particular picture or auction marginally exceeds expectations there are boosters ready to jump in with stories about market take-off, but in context the market is still in the gutter. You can get an absolute masterpiece for a tenth the cost of a Basquiat, a significant museum-quality picture for a tenth the cost of a central London townhouse, and a pretty good entry-level picture for no more than the price of an annual travelcard in London. Head to Sotheby's and Christie's this week for bargains galore!

Sotheby's sale is strong, with some splendid Northern portraits that are much to my taste and a beautiful Murillo Ecce Homo (£2m-3m). But my favourites were a couple of Italian baroque pictures. The strong artistic culture of that time maintained extraordinarily high standards, taking for granted the technical achievements of the Renaissance and trying to get ahead in swagger and bombast. There's a wonderful little mythological picture of Bacchus and Ariadne by Francisco Solimena at Sotheby's estimated at just £300k-£400k. It's a virtuoso little showpiece and I love it. But my favourite is Castiglione's Pagan Sacrifice (£400k-£600k), an incredible picture that I've wanted to see in the flesh ever since I came across it in an old catalogue years ago. It didn't disappoint; one his best pictures. Castiglione can be sloppy, but this one is controlled and the colouration is fabulous.

Christie's has got pobably the most significant work of art in André Beauneveu's marble Lions ('estimate on request'), a remarkable rediscovery from the tomb of Charles V. My favourite of their pictures is a triptych by the Master of the Antwerp Adoration (£600k-£800k), a delightfully inventive creation with wonderful monkey-like faces. Dutch pictures are thin on the ground this year, but I liked this superior Jan Steen Boors playing a game of beugelen (£800k-£1.2m). There's a bargain basement still life, too: a flower piece from the studio of Ambrosius Bosschaert I (£80k-£100k). It's fine quality, and if it was just enough better to lose the 'studio' attribution it would be ten times as much.

The drawings viewings are the biggest draw for me. Museums can't keep old master drawings on display, so you have to grab every chance you can to see them. Sotheby's has an exceptional Canaletto. I get a bit jaded by vedute, but this drawing has it all. Well worth the £2.5m-£3.5m estimate. Prices fall away rapidly below the very first rank. There's an intriguing and wonderful  drawing from Rubens's workshop that's been reworked by the man himself estimated at just 1% of the Canaletto, and a beautiful small Poppi St John the Baptist and a young standing man (£20k-£30k).

Sometimes estimates don't give you much clue, and old master drawings are especially hard to predict. Christie's has taken a cautious approach. I hate it when the tease me into thinking even I can afford something fabulous. The opening lot, Timoteo Viti's The Massacre of the Innocents is surely in a higher league than its £25k-£35k estimate. A wonderful Ribera, above, is estimated at £80k-£120k. It's interesting to compare to Goya, who would be worth ten to twenty times as much. There's a lot to like in a strong sale, including particularly good English drawings. I loved the well-preserved Romneys. But my absolute favourite is Giuseppe Cades's Portrait of the princes Camillo and Francesco Borghese as young boys (below). The £20k-£30k estimate is no guide to its quality, and possibly not much guide to its value either.

If some things are relatively under-rated, I ought to tell you what I think's over-rated too. I heartily disliked the Frans Hals Two Fisherboys (Christie's, £1m-£1.5m). My first thought was Norman Rockwell. Technical analysis shows that it really is old, and Claus Grimm - whose scholarship I revere - thinks it's right. I think it's an awful picture, even if it's an awful picture by Frans Hals. The estimate is too high for a wrong 'un, but surely far too low for an authentic Hals. We'll see. I don't believe the Christie's 'attributed to Rembrandt', either. It's 'estimate on request', but I thought it a weak picture that doesn't rise above any number of competent portraits in his late style.

Turner's Ehrenbreitstein at Sotheby's is unquestionably 'important' (£15m-£25m), but it leaves me cold. I don't care for Turner's figures, and there are too many here. I can admire it, but can't love it.

I'll say more about the day sales next week when I write up the results, but lots of minor treasures there too. Let me end on a high note, with a masterpiece from the start of the Western artistic tradition. This attic red-figured pelike is attributed to the Carpenter Painter, one of the best painters from the best period of Greek vase painting. It's reconstucted from fragments, but the main painted areas seem to be original. Can you believe it's estimated at just £80k-£120k?